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Nov 1, 2013 (3 years and 11 months ago)

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Oxidation of
GaN

for light conversion:

ab

initio

thermodynamic
modelling


Adam J. Jackson
,
Duncan
Allsopp
,
Aron

Walsh

Doctoral Training Centre, Centre for Sustainable Chemical Technologies, University of Bath, BA2 7AY, UK.

E
-
mail:
a.j.jackson@bath.ac.uk
: URL:
http://www.bath.ac.uk/csct


Benefits of improved LEDs

Reduced energy consumption is widely seen as an essential goal to
prevent a global environmental and energy crisis. Lighting makes up
19% of global energy consumption, and it is anticipated that LEDs
will be able to offer around a 90% energy reduction compared with
conventional incandescent lighting.
I
t
is estimated that over 2 billion
people do not have access to affordable, efficient
energy*. Solar
-
powered LED lanterns are currently being trialled in the developing
world, boosting health and education as well as reducing
dependence on kerosene.



Background


GaN

in LEDs


















REFERENCES

Choi, J. H.
et al
.
(2011);
“Nearly single
-
crystalline
GaN

light
-
emitting diodes on amorphous glass substrates”;
N. Photon.
5
, 763
-
769

Goldemberg
, J., and
Johansson, T.B. (2004).
“World Energy Assessment: Overview, 2004
Update”;
United Nations Development Program, United
Nations
Department of
World
Energy Council, New
York

U.S
. Dept. of
Energy, (2010). “Energy Savings Potential of Solid
-
State Lighting in General Illumination Applications 2010 to 2030”

Walsh, A.,
Catlow
, C.,
Miskufova
, M. and
Sokol
, A. A. (2011). “Electron and hole stability in
GaN

and
ZnO
”;
J. Phys.:
Condens
. Matter
23
, 33217

Photo:
Rouzbeh

Delavari

http://www.flickr.com/photos/rouzwawi/2667783454/


lm/W

Operating
time

(
hrs
)

Typical price ($)

Incandescent

13

1,200

0.60

Halogen

15

2,500

3.00

Compact
Fluorescent

55

10,000

3.00

Mercury

Vapour

38

24,000

15.00

Home LED bulb*

65

>25,000

15.00

Experimental LEDs

100+

???

???

U.S. Dept. of Energy (2010) except for *, based on advertised products on
Amazon.com


P
roblems


GaN

is

grown

on

expensive

sapphire

substrate


Oxygen

contamination

during

vapour

deposition


Indium

for

alloying

is

rare,

valuable

and

in
-
demand


Phosphor

coating

is

inefficient















GaN

is a wide
-
bandgap

semiconductor,
notably used in high
-
efficiency bright
blue LEDs. By doping and alloying with
other materials, the semiconducting
properties can be tuned for specific
applications, using In for blue LEDs,
while Al can be added to to push
emitted light into the UV spectrum.

Currently white LEDs are produced
by adding
a phosphor
coating, which shifts
and
spreads
the frequency range of
emitted light
. This comes at an efficiency cost, but opens a
wider range of applications.

Opportunities


Oxidation

is

a

predictable,

controllable

reaction


Oxidation

causes

changes

in

semiconductor

behaviour


Recent

work

shows

that
:


O
xide

substrates

are

possible*


Oxide

semiconducting

properties

are

related**



*Choi
et al

(2011)

**Walsh
et al

(2011)

Methods


Ab
-
initio
, all
-
electron DFT with GGA (
PBEsol
) as
implemented by FHI
-
AIMS package


Periodic boundary conditions used to study bulk solid and
surfaces using slab model


Thermodynamic modelling used to predict relationships
between extent of oxidation, reaction conditions and
electronic properties

*
Goldemberg

et al

(2004)